It is known in the art that the addition of various particulates to activated sludges can increase the biological oxidation rate and the settleability of the sludge. In the past, various particulate matter has been added to the sludge, including fly ash, U.S. Pat. No. 3,342,731; powdered coal, U.S. Pat. No. 3,300,403; sludge coke, U.S. Pat. No. 3,275,547; clay, U.S. Pat. No. 2,769,750; silicate gel, U.S.S.R. Pat. No. 255,138; iron floc, U.S. Pat. No. 3,763,038; limestone powder and cement, U.S. Pat. No. 3,226,319. Some of these processes recognize that the particulate matter itself, which is a waste disposal problem, may be conveniently handled in this manner while improving the biological oxidation process rate and the handling of the sludge. However, it is clear that none of these processes recognizes that spent cracking catalyst may be used in a similar manner. In fact, because of the known sensitivity of bacteria, it would be expected that waste materials of this sort, i.e. with high metals content, could not be added to the activated sludge.